The measurement of blood pressure is an important tool for the medical professional. The term "blood pressure" is a relative term whose precise meaning depends very much on the method used. Blood pressure is the force exerted by the blood against the inner walls of the blood vessels. It is determined by the flow of blood and the resistance to that flow.
Blood pressure is comprised of three parts: the systolic, diastolic, and mean blood pressure. Systolic pressure is the maximum arterial pressure. Diastolic is the minimum arterial pressure. Mean blood pressure is the static pressure that is equivalent to an average pressure. It is found by dividing the area under a single pulse wave by the width of the pulse.
The aortic pressure pulse rises abruptly with aortic ejection and then falls smoothly to the point of the dicrotic notch. The dicrotic notch is attributed to a reflected wave from the recoil of the blood column against the closed aortic valve.
There are several factors that influence arterial blood pressure. They are cardiac output, elastic recoil of the aorta and large arteries, peripheral resistance, volume of blood in the arterial system and viscosity of the blood. Changes in any of the five factors alter either systolic pressure, diastolic pressure, or both pressures.
Devices to measure blood pressure are classified as either direct or indirect. Devices utilizing direct methods all include introducing a pressure sensing element into the blood stream. Because direct methods are invasive, devices utilizing direct methods of measuring blood pressure are impractical for routine clinical examination. Unfortunately, such devices also require a high level of technical skill for the operator. Accordingly, a number of devices utilizing non-invasive indirect methods have been developed. Even though these devices only provide approximate values for intravascular pressure, they are used extensively because physicians need to make routine measurements of blood pressure.
Sphygmomanometry is perhaps the most common type of indirect blood pressure measurement. Sphygmomanometry involves the arrest of flow down an axial artery by the application of a pneumatic cuff. The pressure inside the cuff is manometrically registered. The cuff should meet basic design requirements. Good friction contact is maintained between the cuff and the skin to aid in the constraint of longitudinal tissue motion. The cuff is wide enough to transmit pressure to the artery and the cuff bladder completely encircles the arm.
With sphygmomanometry there must be a means for detecting cessation and onset of blood flow past the cuff as it is inflated or deflated. Monitoring the distal pulse with the finger is one such means. This is, however, insensitive and subjective. Accordingly, more sensitive and objective methods including microphone-amplifier recording, visible capillary refilling or pulsation, plethysmographic pulsation detection, and mercury-in-rubber pulsation detection have been tested. Most notably, auscultation, transcutaneous ultrasonic detection of blood flow, transcutaneous ultrasonic detection of arterial wall movement, and oscillometric pulsation detection are used.
There are two principal ultrasonic methods used in measuring blood pressure. Both methods, however, use a cuff which encircles the limb. In one method, motion of the arterial wall is sensed. In the other, the flow of the blood itself is measured using a Doppler blood flow meter.
Because the walls of the artery beneath an occluding cuff experience a characteristic motion during deflation, it is possible to identify these movements with the first method of ultrasound detection. Two small piezoelectric elements are used. One emits ultrasound and the other detects the ultrasonic echo reflected from the underlying artery. As cuff pressure passes systolic and diastolic pressure, characteristic transitions in the ultrasonic signal are detected.
With the other ultrasonic method, a Doppler blood-flow transducer is placed on the skin over an artery distal to the cuff. When the artery is open, the pulsatile Doppler flow signal is heard or recorded graphically. When the artery is occluded by the cuff, the flow signal disappears. Systolic pressure is read on the first appearance of flow when cuff pressure is decreased During cuff deflation.
The flush method uses an elastic bandage and a limb-encircling cuff. Starting from the tip of the extremity and proceeding to the trunk, a limb is wrapped with a tight elastic bandage such that all the blood is squeezed from the limb. A cuff is then applied just above the trunk end of the bandage and inflated to a high pressure. The bandage is then removed. The opposite undrained limb is then placed beside the blanched member and both are examined in a bright light. Cuff pressure is reduced slowly, and as it passes systolic pressure, blood enters the member and it flushes red. At that particular instant, cuff pressure is read as systolic. There is no indication when cuff pressure is at diastolic pressure or at mean pressure.
Auscultation is listening to sounds that occur within the body. These sounds are known as the Korotkoff sounds in honor of the Russian physician who first proposed the method in 1905. To obtain systolic and diastolic pressure with the auscultatory method, the brachial artery is located and the receiver of a stethoscope placed over it. Cuff pressure is then quickly raised to a point well above systolic pressure. Cuff pressure is then reduced slowly while the observer listens to the arterial sounds. As cuff pressure falls below systolic pressure, a spurt of blood passes under the cuff and a sound is heard in the stethoscope. The cuff pressure at which this sound occurs indicates the systolic pressure. As cuff pressure continues to fall, the sounds become louder, then softer, then very loud, then they become muffled and disappear. Most physicians read the point where the sound disappears as diastolic pressure. However, if the sounds continue to an abnormally low point, physicians use the point where the sound becomes muffled.
In the oscillometric method, variations in amplitude of the blood pressure oscillations are used to identify systolic and mean pressures. There are two components to pressure in a cuff, a static pressure component and a dynamic component. The static component is due to the pressure exerted by the cuff on the limb of body. The dynamic component is due to the pulsation of blood pushing on the cuff. When utilizing the oscillometric method, it is necessary to employ some form of amplification to monitor the small changes in the amplitude of the dynamic component.
In the oscillometric method, cuff pressure is first raised quickly to a point well above systolic pressure where the cuff completely occludes the underlying artery throughout the cardiac cycle. Even though the artery is completely occluded, blood pulsates against the upper edge of the cuff which nevertheless results in small amplitude oscillations on a cuff pressure indicator.
Cuff pressure is then reduced slowly. When cuff pressure falls below systolic pressure, a spurt of blood flows in the artery and the cuff pressure oscillations become larger. As the cuff pressure is further reduced, the oscillations reach a maximum. This maximum corresponds to the maximum change in artery wall dimensions when the heart opens the artery and when the cuff forces the artery closed again on each heart stroke. A further decrease in cuff pressure, therefore, results in a more continuously open artery and the amplitude of the dynamic cuff pressure decreases. The point where amplitude oscillations begin to increase is the point at which systolic pressure is read. The point of maximum oscillation is the mean arterial pressure.
There is, however, no obvious change in cuff pressure oscillation when cuff pressure passes diastolic pressure. Because of this, some physicians have selected diastolic pressure to be the cuff pressure when the oscillations attain a preselected ratio of the maximum amplitude. The ratio is usually chosen to be around 0.8. It has also been assumed that diastolic blood pressure can be obtained from cuff pressure at the point of medium cuff-pressure perturbation.
The methods described above are all manual. These methods all require someone listening and watching to detect the blood pressure. Accordingly, automatic devices capable of reading blood pressure have been developed. The advantages of automatic monitors include ease of use, lower skill level needed by operator, and the elimination of human error in listening for sounds. Automatic, non-invasive blood pressure monitors of this type work either by auscultation or oscillotonometry.
Monitors using auscultation have been available for a number of years.
The first automatic oscillotonometer, on the other hand, was described by Yelderman and Ream in 1977. It consisted of a limb cuff inflated above systolic pressure. Transducers then sensed changes in cuff pressure as the cuff slowly deflated. The first pressure impulse was recorded as systolic pressure; the lowest cuff pressure at which oscillations were maximum was recorded as mean pressure; and the last recorded beat was taken as diastolic pressure. A microprocessor controlled the frequency of recordings and displayed the measurements. The device also included circuits capable of rejecting artifacts produced by patient movement or extraneous pressure on the cuff. Monitors of this type are commercially available.
U.S. Pat. No. 3,903,872 issued to Link in 1975 discloses a single arm cuff for detecting systolic and diastolic blood pressure. The method operates on the principle that pressure applied adjacent to a blood vessel can be plotted against a time derivative of the observed cuff pressure.
U.S. Pat. No. 4,009,709 issued to Link et al. in 1977 discloses a method for detecting systolic pressure. The method uses a conventional arm cuff and an appropriate sensing device to determine the maximum peak pulse amplitude. The systolic pressure is read when the pressure on the cuff is increased until the peak pulse amplitude reading is one half the maximum value.
U.S. Pat. No. 4,651,747 and U.S. Pat. No. 4,664,126 issued to Link in 1987 disclose using a waveform to determine systolic and diastolic pressure. The systolic pressure is determined using the pressure where one-half the maximum pulse amplitude occurs. The diastolic pressure is determined using the slope of the diastolic portion of the pulse curve. Link further shows a method to calculate the area under the curve to obtain the mean arterial pressure.
U.S. Pat. No. 4,729,382 issued to Schaffer et al in 1988 discloses using a two cuff method. The device includes a pressure differential sensor on each cuff and a third sensor to read the static pressure on the proximal cuff. The cuffs are inflated above the point where the second cuff (proximal cuff) occludes blood flow. The sensor in the first cuff (distal cuff) senses no arterially induced pressure pulsation amplitudes. Then deflation begins in both cuffs. When the first cuff detects a pulse, the static pressure sensor connected to the second cuff (proximal cuff) is read to record the systolic pressure. The diastolic pressure is read when the signal on the second cuff (proximal cuff) reaches a steady state.